Because Arthur Dent keeps killing Agrajag over and over again and he reincarnates into various people and creatures that then are killed by Arthur. The pot of petunias is one such reincarnation.
The entire character arc is basically Douglas Adams taking out his frustration with Jaguar drivers in traffic.
It is an incredible set of books. You can return to it years later and find jokes and references that you missed in first... few hundred readings (don't judge me :) ).
